A 5870 is more than a match at 1920x1080 so you really wouldnt need the extra beef unless youre using a 120Hz monitor and want those triple digit frame rates in all your games. But yeah, nothing wrong with MOAR HORSEPOWER!
haha yeah man. Well I just really want to crank my settings and hold a nice framerate. I also want to be ready for future games. Here is my PC specs
intel Core2 Quad Q9450 3.4GHz
4gb Corsair Dominator
XFX 5870
Biostar Tpower 145 motherboard
Windows 7 Ultimate 64bit
Thermaltake toughpower 750w
Id go for it. Thats a good PSU and enough grunt to push a pair of 5870's. $170 is a good price on those things especially as they seem to really be sought after for that Bitcoin bullshit or whatever that is. It would be enough muscle to last for a good while.
Drivers could be an issue. I know AMD was having some trouble getting good drivers out for 5800's in Crossfire but I think most of that has been ironed out now. Might want to look into that a little though before pulling the trigger.
I would make sure that you can test it somehow before you hand him the cash. I've heard plenty of bad stories from Craigslist scams.
Not saying that your experience will be the same, I'm sure there's plenty of good stories, but don't have blind faith.
Also, I would go for it, I've seen really great things about CF5870's.
If he tells you that he can't test it for you because he's sold all his other components and has no desktop now or some other excuse, walk away. He may also tell you to bring your computer in the hopes that you decide it's too much of a hassle and just buy it on faith. Call his bluff if so.